A vehicle transmission can deliver mechanical power from an engine to the remainder of a drive system, typically fixed gearing, axles, and wheels. A transmission allows some freedom in engine operation, usually through alternate selection of five or six different drive ratios, a neutral selection that allows the engine to operate accessories with the vehicle stationary and reverse. Transmission gear selection typically allows power from the engine to be delivered to the rest of the drive system with a ratio of torque multiplication and speed reduction and with a ratio of torque reduction and speed multiplication known as overdrive.
An electric generator can transform mechanical power from the engine into electrical power, and an electric motor can transform that electric power back into mechanical power at different torques and speeds for the remainder of the vehicle drive system. This arrangement allows a continuous variation in the ratio of torque and speed between the engine and the remainder of the drive system, within the limits of the electric machinery. An electric storage battery used as a source of power for propulsion may be added to this arrangement, forming a series hybrid electric drive system.
The series hybrid system allows the engine to operate relatively independently of the torque, speed, and power to propel a vehicle, so as to be controlled for improved emissions and efficiency. This system also allows the electric machine attached to the engine to function as a motor to start the engine and allows the electric machine attached to the remainder of the drive train to act as a generator, recovering energy into the battery by regenerative braking. However, a series electric drive requires that the electrical machinery be sufficiently sized to transform all engine power from mechanical to electrical form and from electrical to mechanical form, and useful power is lost in this double conversion.
A power split transmission can use what is commonly understood to be “differential gearing” to achieve a continuously variable torque and speed ratio between input and output without sending all power through the variable elements. One form of differential gearing may constitute a planetary gear set. In fact, planetary gearing is usually the preferred embodiment employed in differentially geared inventions, with the advantage of compactness and different torque and speed ratios among all members of the planetary gear set. However, it is possible to construct this invention without planetary gears, as by using bevel differential gears or other differential gears.
A simple electrically variable transmission can use differential gearing to split power from the engine into two power paths to the wheels of the vehicle; an electro-mechanical path through a pair of electric machines or motor-generators and an all-mechanical parallel path that is fixed ratio or alternatively selectable. Electrically variable transmissions may form input-power-split, compound-power-split, or output-power-split configurations.
An electrically variable transmission in a vehicle can simply transmit mechanical power. To do so, the electric power produced by one motor-generator balances the electrical losses and the electric power consumed by the other motor-generator. A hybrid electrically variable transmission system in a vehicle includes an electrical storage battery, so the electric power generated by one motor-generator can be greater than or less than the electric power consumed by the other. Electric power from the battery can sometimes allow both motor-generators to act as motors, especially to assist the engine with vehicle acceleration. Both motors can sometimes act as generators to recharge the battery, especially in regenerative vehicle braking. Electric power from the battery also allows engine starting with the transmission system.
One of the most successful substitutes for the series hybrid transmission is the variable, two-mode, input-split, parallel-hybrid electric transmission. Such a transmission utilizes an input means to receive power from the vehicle engine and a power output member to deliver power to drive the vehicle. First and second motor-generators are connected to an energy storage device, such as a battery, so that the energy storage device can accept power from and supply power to the first and second motor-generators. A control means regulates power flow among the energy storage devices and the motor-generators as well as between the first and second motor-generators.
Operation in a first or second mode may be selectively achieved by using clutches. In one mode, the output speed of the transmission is directly proportional to the speed of one motor-generator, and in the second mode the output speed of the transmission increases along with the speed of the other motor-generator.
In some embodiments of the variable, two-mode, input-split, parallel-hybrid electric transmission a planetary gear set is selectively employed for torque multiplication. In addition, some embodiments may utilize three torque transmitting mechanisms—two to select the operational mode desired of the transmission and the third selectively to disconnect the transmission from the engine.
As those skilled in the art will appreciate, a transmission system using a power split arrangement may receive power from two sources. However, the prior art does not include any practical gear schemes with an input-split operating mode and more than two compound split operating modes.